It is known that a subscriber line interface circuit is a device intended to connect a subscriber's line to a telephone exchange. One of the functions of a line interface circuit is the supply of a d.c. current to the subscriber's line. An electronic subscriber line interface network of the type described in the opening paragraph is disclosed in U.S. Pat. No. 4,275,277. This line interface circuit is of such a construction that it operates in practice as a transmission bridge which is connected to the d.c. voltage source and has a resistance which is in a perfect balance with respect to the two wires of the subscriber's line, this balance being independent of the longitudinal currents which may happen to occur on the subscriber's line. By controlling the sum current and/or the impedance through which this sum current flows, it is possible to adjust the resistance of the transmission bridge of the connecting network to a specified value without modifying its balance.
In order to prevent a large amount of a power to be dissipated in the amplifiers which supply the d.c. current to the subscriber's line and to facilitate the integration of the line interface circuit, it was recommended in U.S. Pat. No. 4,292,735 to arrange two load resistors between the two supply terminals of the amplifiers and the two terminals of the supply source. These two resistors have for their object to acquire a portion of the d.c. voltage drop to be produced by the line interface circuit. In said patent these load resistors are dipoles the impedance of which depends on the current flowing through them. When, however, a longitudinal current is accidentally produced in the subscriber's line there flows through one of these dipoles the transversal current produced by the connecting circuit, increased by the longitudinal current while through the other dipole there flows the transversal current decreased by the longitudinal current. The result is that these two dipoles do not produce the same voltage drop and that for a certain intensity of the longitudinal current one of the dipoles produces a voltage drop of such a high value that correct operation of the amplifier to which it is connected is prevented. Thus, when load resistors in the form of dipoles are used, the proper operation of the connecting network is disturbed when there is a longitudinal current which exceeds the transversal current supplied by the line interface circuit by 20 to 30%.
It is an object of the invention to avoid these limitations in the good performance of the line interface circuit by using load resistors the value of which is substantially independent of the longitudinal current.